1. Field of the Invention
This invention relates to a bending apparatus having a die and a punch for bending a sheet (i.e., a workpiece).
2. Description of the Related Art
Japanese Kokai Patent Publication H2-112826 discloses a bending apparatus for bending a composite (laminate) metal sheet comprising an outer and an inner metal sheet and a synthetic resin layer interposed between the two metal sheets. The purpose of this apparatus is to preclude a phenomenon of bending due to deviation of the outer and inner metal sheets. To meet this end, the apparatus comprises a first die, which has an end valley for constituting a V-shaped groove and is vertically movable, and a second die, which has wall surfaces for constituting the V-shaped groove. When the first die is pushed by the punch to be lowered to a predetermined position, its end valley becomes continuous to the inclined wall surfaces of the second die, thus completing the V-shaped groove. In a first stage of operation a V-shaped bend of short inclined surfaces is formed with the first die, and in the second stage a V-shaped bend of long inclined surfaces is formed with the first and second dies to thereby correct the bend formed in the first stage.
According to this teaching, however, there is a problem in that a flaw is provided in the workpiece along the boundary between the first and second dies.
Japanese Kokai Utility Model Publication H3-14010 discloses a bending apparatus, which, like the above bending apparatus, has a purpose of precluding the phenomenon of bending when bending a composite (laminate) metal sheet. The apparatus features a pair of steel receptacles having respective spherical or semi-cylindrical journals formed on the back side. The surfaces of the steel receptacles, on which the workpiece is set, are located on the outer side of the center of rotation of the journals. In this arrangement, in an initial stage of the bending process the workpiece undergoes the bending process while the steel plate receptacles are moved away from the bending line. Thus, the phenomenon of bending due to the pulling of the workpiece toward the bending line can be basically avoided. In addition, since the amount of friction between the workpiece and the steel receptacles is reduced during the bending process, it is possible to inhibit the generation of flaws due to the friction.
In this case, however, at the end of the bending process a gap is produced between the steel receptacles. Thus, it is impossible to apply back pressure to the outer surface of the workpiece near the radius portion, and it is thus difficult to make the bending radius smaller than a certain value. Further, a flaw is likely to be produced along the borderline between the steel receptacles and the workpiece.
Japanese Kokai Utility Model Publication H2-42718 discloses a bending apparatus which seeks to solve the above problem. This apparatus comprises a V-shaped die having a central V-shaped die groove, a pair of slidable supports of an elastic material provided at the ends of the V-shaped die, and rotary dies provided on the upper end of the slidable supports and having a semi-circular sectional profile. In the initial stage of the bending process, the workpiece is bent about its portion in contact with the punch with the descent of the punch and the rotation of the rotary dies, and at the end of the bending process the V-shaped die is brought into contact with the radius portion. Thus, accurate bending can be obtained.
However, again in this case an entire surface contact cannot be obtained although an end portion of the V-shaped die is brought into contact with the workpiece at the end of the bending process, thus posing the problem that a flaw is liable to be produced in the workpiece.
Japanese Kokai Patent Publication H2-11225 discloses a further bending apparatus, which, like the above bending apparatus, seeks to suppress the phenomenon of bending. To attain this aim, in a first step an elastic block is set in a V-shaped groove of a die for bending with a punch, and a second step of bending is performed with a workpiece reception die, which is made of two different elastic materials having different hardnesses.
However, again in this case the restoration of the elastic materials from deformation is deteriorated with the lapse of time. Therefore, the apparatus is not suitable for continuous operation. In addition, it is thought that the quality of bending is subject to deterioration with the lapse of time.
Further, the width of the V of the die that is required to preclude flaws from being produced in the workpiece is 5 to 6 times the thickness of the workpiece. In the above case, however, it is possible to cope with only a single workpiece thickness.